There's two ways you can do it - the hard way, or the easy way! Personally I would do it the hard way; drive right down the Nicoya Peninsula. You will encounter some amazing scenery and the drive itself will be an adventure. You don't mention when you are visiting, but if it's during the rainy season, this might not be advisable. During the dry season it can be done in 6-8 hours I would say. In the rainy season, take the easy option; get the ferry! Your best bet would be to drive down to Puntarenas and hop on the ferry. It's about a one hour crossing and then another hours drive to Montezuma. Not as exciting, but more functional!

We enjoyed the ferry ride. The coastline along there is so unspoiled and beautiful. Just find out the ferry schedule ahead of time and get there an hour early, so you make the boat. It's pretty cheap, just a few dollars for you and your car.